1. Field of the Invention
This invention relates to medical devices and methods. More specifically, the invention relates to intervertebral disc prostheses.
Back pain takes an enormous toll on the health and productivity of people around the world. According to the American Academy of Orthopedic Surgeons, approximately 80 percent of Americans will experience back pain at some time in their life. In just the year 2000, approximately 26 million visits were made to physicians' offices due to back problems in the United States. On any one day, it is estimated that 5% of the working population in America is disabled by back pain.
One common cause of back pain is injury, degeneration and/or dysfunction of one or more intervertebral discs. Intervertebral discs are the soft tissue structures located between each of the thirty-three vertebral bones that make up the vertebral (spinal) column. Essentially, the discs allow the vertebrae to move relative to one another. The vertebral column and discs are vital anatomical structures, in that they form a central axis that supports the head and torso, allow for movement of the back, and protect the spinal cord, which passes through the vertebrae in proximity to the discs.
Discs often become damaged due to wear and tear or acute injury. For example, discs may bulge (herniate), tear, rupture, degenerate or the like. A bulging disc may press against the spinal cord or a nerve exiting the spinal cord, causing “radicular” pain (pain in one or more extremities caused by impingement of a nerve root). Degeneration or other damage to a disc may cause a loss of “disc height,” meaning that the natural space between two vertebrae decreases. Decreased disc height may cause a disc to bulge, facet loads to increase, two vertebrae to rub together in an unnatural way and/or increased pressure on certain parts of the vertebrae and/or nerve roots, thus causing pain. In general, chronic and acute damage to intervertebral discs is a common source of back related pain and loss of mobility.
When one or more damaged intervertebral discs cause a patient pain and discomfort, surgery is often required. Traditionally, surgical procedures for treating intervertebral discs have involved discectomy (partial or total removal of a disc), with or without fusion of the two vertebrae adjacent to the disc. Fusion of the two vertebrae is achieved by inserting bone graft material between the two vertebrae such that the two vertebrae and the graft material grow together. Oftentimes, pins, rods, screws, cages and/or the like are inserted between the vertebrae to act as support structures to hold the vertebrae and graft material in place while they permanently fuse together. Although fusion often treats the back pain, it reduces the patient's ability to move, because the back cannot bend or twist at the fused area. In addition, fusion increases stresses at adjacent levels of the spine, potentially accelerating degeneration of these discs.
In an attempt to treat disc related pain without fusion, an alternative approach has been developed, in which a movable, implantable, artificial intervertebral disc (or “disc prosthesis”) is inserted between two vertebrae. A number of different intervertebral disc prostheses are currently being developed. For example, the inventors of the present invention have developed disc prostheses described in U.S. patent application Ser. Nos. 10/855,817 and 10/855,253, previously incorporated by reference. Other examples of intervertebral disc prostheses are the LINK® SB Charité disc (provided by DePuy Spine, Inc.) Mobidisk® (provided by LDR Medical (www.ldrmedical.fr)), the Bryan Cervical Disc (provided by Medtronic Sofamor Danek, Inc.), the ProDisc® or ProDisc-C® (from Synthes Stratec, Inc.), and the PCM disc (provided by Cervitech, Inc.). Although existing disc prostheses provide advantages over traditional treatment methods, improvements are ongoing.
One type of intervertebral disc prosthesis generally includes upper and lower prosthesis plates or shells, which locate against and engage the adjacent vertebral bodies, and a low friction core between the plates. In some designs, the core has upper and lower convexly curved surfaces, and the plates have corresponding, concavely curved recesses which cooperate with the curved surfaces of the core. This allows the plates to slide over the core to allow required spinal movements to take place. Some type of movement limiting structure is provided, to prevent the core from slipping out between the plates. Typically, the plates are made of one or more metals, and the core is made of a polymeric substance.
One of the challenges in designing an intervertebral disc prosthesis is to prevent or reduce wear and tear of the core. In many prosthetic discs, the plates are metallic, and the core is made of a polymeric material. Although a core made of a polymeric or other resilient material may last many years, it would be advantageous to have cores that could last even longer, so that patients (especially younger patients) would not be faced with the possibility of repeat surgeries to replace disc prostheses. At the same time, longer-lasting cores should still allow for a desirable range and ease of motion of the two vertebrae about the intervertebral disc prosthesis.
Therefore, a need exists for improved intervertebral disc prostheses. Ideally, such improved prostheses would provide improved resistance to wear and tear while also allowing a desired amount of vertebral movement about the prosthesis. At least some of these objectives will be met by the present invention.
2. Description of the Background Art
A number of exemplary intervertebral disc prostheses are listed above. Published US patent applications 2002/0035400A1 and 2002/0128715A1 describe disc implants which comprise opposing plates with a core between them over which the plates can slide. The core receives one or more central posts, which are carried by the plates and which locate in opposite ends of a central opening in the core. Such arrangements limit the load bearing area available between the plates and core.
Other patents related to intervertebral disc prostheses include U.S. Pat. Nos. 4,759,766; 4,863,477; 4,997,432; 5,035,716; 5,071,437; 5,370,697; 5,401,269; 5,507,816; 5,534,030; 5,556,431; 5,674,296; 5,676,702; 5,702,450; 5,824,094; 5,865,846; 5,989,291; 6,001,130; 6,022,376; 6,039,763; 6,139,579; 6,156,067; 6,162,252; 6,315,797; 6,348,071; 6,368,350; 6,416,551; 6,592,624; 6,607,558 and 6,706,068. Other patent applications related to intervertebral disc prostheses include U.S. Patent Application Publication Nos.: 2003/0009224; 2003/0074076; 2003/0191536; 2003/0208271; 2003/0135277; 2003/0199982; 2001/0016773 and 2003/0100951. Other related patents include WO 01/01893A1, EP 1344507, EP 1344506, EP 1250898, EP 1306064, EP 1344508, EP 1344493, EP 1417940, EP 1142544, and EP 0333990.